Brain aneurysm is a condition characterized by ballooning and expansion of the brain vessel's wall which can potentially rupture and cause bleeding.
Brain is a very energy consuming organ. It weights around 1.5 kg yet takes approximately 15% of total blood flow and 20% of total oxygen consumption. It is supplied by 4 major vessels - two carotid and two vertebral arteries. These arteries form a complex network called "circle of Willis" which supplies the brain by giving off smaller branches. Circle of Willis allows the brain to maintain blood flow in cases of major artery obstruction. It is an evolutional adaptation to keep all brain parts continuously supplied. Technically brain arteries and circle of Willis are located outside of the brain in the subarachnoid space. This space is filled with cerebrospinal fluid.
The flow and the pressure in the brain arteries are very high due to massive demand. Also, the wall of brain arteries lacks external supporting layer (tunica externa or adventitia layer) present in other arteries. Therefore, brain arteries are very prone to aneurysm formation. The weak spots in the artery wall become bulged due to pressure inside of the artery. These small bulges sometimes called blister aneurysms because they can also rupture and cause bleeding. With time this bulge become more and more prominent due to Laplace law. According to this law the greater the diameter of the vessel the greater the tension of its wall given the pressure is constant. Therefore, bigger aneurysms tend to rupture more frequently.
All brain aneurysm are true aneurysms, unlike false aneurysms that may develop in other areas. Brain aneurysms usually develop at sites where the artery branches off.
Blood flow at branching sites is usually turbulent putting additional mechanical stress on arterial wall. Thus, these sites are especially prone to aneurysm formation. High blood pressure (hypertension), smoking, genetic predisposition (familial cases), atherosclerosis are factors contributing to aneurysm formation.
There are three types of brain aneurysms:
Berry or saccular aneurysms – the most common type of aneurysms. These aneurysms develop due to dilation of the part of the artery wall. They usually have rounded shape and may be from several millimeters to several centimeters.
Usually these aneurysms become symptomatic due to rupture or compression of nearby structures. They may also develop due to increased flow and turbulence. If one of the main brain arteries is occluded the others have to compensate through the circle of Willis. Increased flow puts additional mechanical stress on arteries, leading to aneurysm formation. Brain arterio-venous malformations (AVM) also frequently associated with aneurysms. AVM’s divert blood flow from the brain tissue directly to veins. Since this alternative way has very low resistance the flow is very high. Therefore, aneurysms often develop on arteries supplying AVM and on circle of Willis.
Fusiform aneurysms – these aneurysms are dilation of the entire wall of the vessel and resemble a spindle. They also have potential to rupture but in the vast majority of cases they usually cause compression of nearby structures and cause neurological deficit. In some instances, the flow in the aneurysm is slow due to dilation which leads to blood flow stagnation and thrombus formation. These thrombi may dislodge and cause blood flow obstruction leading to ischemic stroke.
Mycotic aneurysms – these aneurysms form due to infection. They are usually associated with infective endocarditis. Infected thrombi with bacteria in it may stuck inside brain vessels. Bacteria will cause inflammation inside the artery wall leading to weakening and aneurysm formation. These aneurysms usually develop on small brain arteries and are very rare.
In the vast majority of cases, brain aneurysms do not cause any symptoms until they rupture and cause hemorrhage. Since brain arteries are located in the subarachnoid space, blood spills into this space causing a subarachnoid hemorrhage.
This type of hemorrhage is associated with sudden onset of severe headache. Some patients refer it as the worst pain of their life. This pain is due to irritation of meninges (brain covers that are very rich with pain receptors).
Blood spilled into subarachnoid space forms clots within minutes after bleeding. These clots may obstruct cerebro-spinal fluid flow and cause acute hydrocephalus. Also starting several days after hemorrhage blood clots break down releasing blood degradation products. This is a very dangerous time period since blood degradation products cause vasospasm - i.e. narrowing of brain arteries. The resultant effect is decreased flow to the brain with ischemic change further aggravating the patient's condition. In some cases, the hemorrhage may be very massive and aneurysm may rupture not only into subarachnoid space but also in the brain itself. Patients may have fall in coma and/or develop neurological deficits.
Aneurysms may cause compression of nearby cranial nerves. Second and third cranial nerves are usually very prone to this type of compression and patients may experience blurry or double vision. Sometimes aneurysms may be gigantic and cause brain compression resulting either in neurological deficit of epilepsy.
Usually a brain computerized tomography (CT) scan is performed and bleeding is confirmed. Sometimes bleeding may be so miniscule CT scan may not detect it. Lumbar puncture might be performed on these patients. If bloody cerebrospinal fluid encountered the diagnosed is confirmed. Below is a CT scan of a patient with subarachnoid hemorrhage (white areas in the brain).
When brain aneurysm is suspected, brain angiography (literally means vessel imaging) is required to demonstrate its presence, size and location. Angiography may be performed on CT (CT angiography or CTA), MRI (MR angiography or MRA) or through catheters placed into brain vessels (Digital Subtraction Angiography). Although DSA is considered as gold standard for detecting brain aneurysm it is an invasive procedure. CTA and MRA nowadays are capable of demonstrating the majority of aneurysms and avoid DSA procedure. Below is the picture of CTA of patient who was referred for spontaneous subarachnoid hemorrhage and was found to have two aneurysms.
Treatment for aneurysms depends on their rupture status. If aneurysm is found incidentally on brain scan usually there is no need for urgent intervention. These cases are done electively. However ruptured aneurysms should be treated as soon as possible to prevent re-bleeding.
There are two main treatment option for brain aneurysms – microsurgical clipping and endovascular treatment. Microsurgical clipping is performed by neurosurgeons and consists of approaching the aneurysm and placing a permanent clip across the neck of aneurysm to obstruct the flow into it. This way aneurysm is effectively excluded from blood circulation. It is a very effective treatment modality.
The rate of complete occlusion is high and the recurrence following microsurgical clipping is very low. Another advantage of surgical intervention is ability to remove blood clots from the brain and decrease the rate of vasospasm and hydrocephalus.
Endovascular treatment is a collective term which includes many various types of interventions that aim either filling the aneurysm or diverting the flow from the aneurysm to reduce the chance of bleeding. These treatments are usually provided either by neurosurgeons of interventional radiologists. The major advantage is avoidance of open brain surgery, yet long term occlusion rates are not as satisfactory as with microsurgical clipping. The decision of treating a particular aneurysm with surgery or endovascular means should be made by a specific experienced team practicing both treatment modalities.